1. Field of the Invention
The present invention relates to the field of improving the electrochemical performances of an alkali metal oxyanion electrode material, and more specifically, to a process for improving the electrochemical performances of an alkali metal oxyanion electrode material having a pyrolitic carbon deposit thereon as well as to the alkali metal oxyanion electrode material obtained therefrom.
2. Description of the Related Art
Alkali metal oxyanions, useful as cathode material, have been shown to present problems relating to electrochemical performances. Undesirable low electronic conductivity is one example of such problem. One significant improvement to the problem of low electronic conductivity of alkali metal oxyanion material, for instance of alkali metal phosphate, has been achieved with the formation of a carbon deposit on the surface of the material. Ravet [e.g., U.S. Pat. No. 6,855,273, U.S. Pat. No. 6,962,666, U.S. Pat. No. 7,344,659, U.S. Pat. No. 7,815,819, U.S. Pat. No. 7,285,260, U.S. Pat. No. 7,457,018, U.S. Pat. No. 7,601,318, WO 02/27823 and WO 02/27824)] has proposed using an organic carbon precursor that is pyrolysed onto the cathode material or its precursors, thus forming a carbon deposit, to improve electrical field at the level of the cathode particles.
In the specific case of carbon-deposited lithium iron phosphate, referred to as C—LiFePO4, several processes have been used to obtain the material, either by pyrolysing a carbon precursor on LiFePO4 powder or by simultaneous reaction of lithium, iron and PO4 sources and a carbon precursor. For example, WO 02/27823 and WO 02/27824 describes a solid-state thermal process allowing synthesis of C—LiFePO4 through following reaction:Fe(III)PO4+½Li2CO3+carbon precursor→C—LiFe(II)PO4 
In which the carbon precursor is an organic material that forms a carbon deposit through pyrolysis while generating reducing gases that efficiently reduce the iron (III).
Such process has been scaled-up to produce large quantity of battery grade cathode material. However, as “one-pot” solid-state process involving numerous simultaneous chemical, electrochemical, gas-phase, gas-solid reactions, sintering and carbon deposition, C—LiFePO4 electrochemical properties are dependent on numerous parameters such as surface properties, wettability, surface area, porosity, particle size distribution, water-content, crystal structure, as well as on the raw materials chemistry, reactor feed rate, flow of gas, etc. In consequence, undesirable fluctuations on cathode material properties, especially electrochemical capacity (mAh/g), have been observed.
Problems remain to find a simple and cost-effective chemical treatment allowing to up-grade quality and consistency of commercial products for battery applications.